fix: handle PS2 prompts that require prompt-expansion

[reubeno]

Tools like starship customize PS2 in addition to PS1; the same logic used to expand and evaluate PS1 should apply to PS2 as well.

[github-actions]

Test Results

  2 files    9 suites   1m 9s ⏱️
499 tests 499 ✅ 0 💤 0 ❌
986 runs  986 ✅ 0 💤 0 ❌

Results for commit fe408e0.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.39 μs	3.42 μs	0.03 μs	⚪ Unchanged
instantiate_shell	59.85 μs	67.26 μs	7.40 μs	🟠 +12.37%
instantiate_shell_with_init_scripts	30751.87 μs	30956.51 μs	204.64 μs	⚪ Unchanged
parse_bash_completion	2797.06 μs	2802.28 μs	5.22 μs	⚪ Unchanged
parse_sample_script	4.20 μs	4.19 μs	-0.00 μs	⚪ Unchanged
run_echo_builtin_command	91.81 μs	92.20 μs	0.39 μs	⚪ Unchanged
run_one_builtin_command	111.39 μs	109.53 μs	-1.87 μs	⚪ Unchanged
run_one_external_command	1963.56 μs	1979.21 μs	15.65 μs	⚪ Unchanged
run_one_external_command_directly	1001.66 μs	1010.53 μs	8.87 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/shell.rs	🟢 78.66%	🟢 78.59%	🔴 -0.07%
Overall Coverage	🟢 73.56%	🟢 73.56%	⚪ 0%

Minimum allowed coverage is 70%, this run produced 73.56% |

Minimum allowed coverage is 70%, this run produced 77.1%